3.3.4Organism Level Systems

Responses to Extremes — OCR Gateway GCSE Combined Science A J250 - Biology Revision Notes & Topical Questions

5 min read 9 parts Lesson 13 of 13

📚 OCR Gateway GCSE Combined Science A J250 - Biology Hub / Topical Revision

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Part 1 of 7When homeostasis is stretched

Homeostatic mechanisms work well under normal conditions, but extreme situations can overwhelm them. Prolonged heat or cold, severe water shortage, or very intense exercise can push the body beyond its ability to maintain a constant internal environment, leading to illness. The key is to understand which mechanism is under strain and what happens when it fails.

Part 2 of 7Extreme heat

In very hot weather, the body loses heat by sweating and vasodilation, but if the temperature or humidity is very high, or if heat is produced faster than it can be lost, the core temperature rises. Heat exhaustion includes heavy sweating, pale skin, headache, nausea and a fast pulse.

Heat stroke

If untreated it can become heat stroke, in which the temperature regulation fails; sweating may stop, the skin becomes hot and dry,

and body temperature exceeds 40 degrees Celsius.

Enzymes begin to denature, and organs can be damaged.

Treatment is to cool the person down, move them to shade and give fluids.

Water loss in sweat lowers the blood volume,

so ADH release rises and the kidneys reabsorb more water,
but if water is not replaced the person becomes dehydrated.

Part 3 of 7Extreme cold

In cold conditions, vasoconstriction and shivering conserve and generate heat, but if the heat loss exceeds production, core temperature falls below about 35 degrees Celsius: hypothermia.

The metabolic rate falls, enzyme activity slows, the heart beats slower, and the person becomes confused and drowsy and may lose consciousness.

Cold water is especially dangerous because water conducts heat away much faster than air.

Wet clothing, wind, and exposure make the situation worse.

Very cold extremities can suffer frostbite, as blood vessels in the fingers and toes constrict to protect the core and tissue freezes.

Babies and elderly people have a greater risk because they have less effective control and a larger surface area to volume ratio (babies).

Part 4 of 7Exercise and water balance

During intense exercise, the rate of respiration in muscle increases, producing heat and carbon dioxide.

Core temperature rises, so sweating and vasodilation increase.

Blood carbon dioxide rises, which the brain detects, raising the breathing rate and heart rate.

Muscles use glucose quickly, so glucagon and adrenaline help maintain blood glucose.

If sweating is heavy and the water is not replaced, blood becomes concentrated, ADH rises and urine volume falls.

Athletes who sweat for hours lose salt as well as water; drinking only water can dilute the blood and cause confusion.

Sports drinks with salts replace both.

During extremely long exercise glycogen stores run out, causing fatigue.

Extreme	Main responses	Danger if overwhelmed
Heat	Sweating, vasodilation	Heat stroke, dehydration
Cold	Shivering, vasoconstriction	Hypothermia, frostbite
Water shortage	ADH, concentrated urine	Dehydration, kidney damage
Heavy exercise	Faster breathing and heart rate, sweating	Exhaustion, low glucose

Part 5 of 7Applying knowledge in exams

Extended questions often describe a case, such as a marathon runner or a mountain walker, and ask you to explain what is happening.

Name the stimulus and the receptors, the coordination centre, the effectors and the response, and say whether the feedback is working or failing.

Give figures from any data provided, for example core temperature rising from 37 to 40 degrees.

Use the idea of enzymes denaturing to explain why heat stroke is life-threatening and the idea of slowed reactions to explain why hypothermia is dangerous.

Link the case to the kidney and ADH if water balance is involved.

Part 6 of 7Case studies and numbers to know

Some numbers help to anchor answers.

Case studies and numbers to know

Normal core body temperature is about 37 degrees Celsius; hypothermia is diagnosed below about 35 degrees, and heat stroke is above about 40.

Blood glucose is normally 4 to 6 mmol per dm³.

A person can survive for weeks without food but only a few days without water, because water balance affects every cell.

Mountaineers also face low oxygen at altitude: breathing and heart rates rise at first,

and over days the body makes more red blood cells to carry oxygen,
which links this lesson to blood.

Case studya long-distance runner collapses at a hot event with a core temperature of 41 degrees Celsius, hot dry skin and confusion.

The explanation is that heat produced by respiration in the muscles exceeded the heat lost; sweating has failed, partly because of water loss; enzymes in the brain are being affected.

The correct first aid is to cool the person rapidly, for example with cold wet cloths, and to give fluids if conscious.

A second case: a walker lost in snow, wearing wet clothes, shows slow movements and confusion because the core temperature has fallen to 33 degrees; treatment is to warm slowly and dry the person.

Practising such cases helps you apply the same pattern of reasoning to any new example.

Part 7 of 7Exam-style questions

Explain why a person with heat stroke is in danger of dying. [3]3 marks
A hiker falls into a cold river. Explain why the risk of hypothermia is high. [3]3 marks
A marathon runner drinks only water for several hours in the heat. Suggest why she may become confused. [3]3 marks

Key terms

Term	Meaning
Hypothermia	Core body temperature that falls dangerously low.
Heat stroke	A life-threatening condition in which body temperature control fails in heat.
Dehydration	Loss of too much water from the body.
Frostbite	Freezing of tissue, usually in fingers and toes.
Vasoconstriction	Narrowing of skin arterioles to reduce heat loss.
Metabolic rate	The rate of chemical reactions in the body.
Exhaustion	A state in which energy stores or body systems cannot keep up with demand.

Avoid these mistakes

Describing hypothermia as a high body temperature.

Saying that in heat stroke the person sweats more; sweating often stops.

Forgetting that sweating loses salts as well as water.

Not linking the effect to enzymes and cells when explaining the danger.

Thinking shivering is a voluntary action; it is a reflex response.

Last updated: 20 October 2026